Biomarkers are a group of compounds, primarily hydrocarbons, found in oils, rock extracts, Recent sediment extracts, and soil extracts. Biomarkers consist of complex organic molecules whose chemical structures can remain largely unchanged during the processes of diagenesis and oil generation. This often allows them to be traced back to the original molecules in once living organisms. For this reason they are often called molecular fossils. They are structurally similar to, and are diagenetic alteration products of, specific natural products (compounds produced by living organisms).
Typically, biomarkers retain all or most of the original carbon skeleton of the original natural product, and this structural similarity is what leads to the term "molecular fossils".
Some biomarkers are prolific in many organisms while others are only found in specific types of organisms and hence these can be used as markers of source input. Additionally, some biomarkers do not appear until their parent organism has evolved, and therefore the presence of these compounds can be used as an age diagnostic marker. Similarly, some biomarker structures are more stable in certain depositional environments than others and hence their abundance can be used to infer depositional environments. Also, subtle chemical transformation from one chemical structure into another can occur due to thermal stress and hence the ratio of these biomarkers can be used as a maturity marker.
Biomarkers have a variety of applications in petroleum exploration. For example:
- When samples of oil and candidate source rocks are available, biomarkers can be used to make oil-source rock correlations, or
- When samples of candidate source rocks are not available, the biomarker distribution in an oil can be used to infer characteristics of the source rock that generated the oil without examining the source rock itself. Specifically, biomarkers in an oil can reveal (1) the relative amount of oil-prone vs. gas-prone organic matter in the source kerogen, (2) the age of the source rock, (3) the environment of deposition as marine, lacustrine, fluvio-deltaic or hypersaline, (4) the lithology of the source rock (carbonate vs. shale), and (5) the thermal maturity of the source rock during generation (e.g., Peters and Moldowan, 1993). Such data may be key inputs to effective basin modeling of a prospect or block.
Petroleum Biomarkers Indicative of Source Rock Organic Matter Input and Depositional Conditions
Below are a few examples of oil biomarker parameters that provide information about the depositional environment of the source rock and the origin of the organic matter in the source rock.
To characterize charge risk, these biomarker parameters can be used in a variety of innovative ways. For example, specific biomarker parameters can be calibrated against specific kerogen quality parameters in a given basin. Then, the biomarker ratios are measured in an oil sample from the basin, and the values are projected onto calibration curves to quantitatively predict characteristics of the source rock. This approach, pioneered by the founders of OilTracers, allows explorationists to assess whether an oil was generated primarily from an oil-prone or gas-prone organic facies (Dahl et al., 1994; McCaffrey et al., 1994). The information gained from oil biomarkers (source type, age, maturity, kerogen quality) when integrated into a basin model has substantial economic impact because it provides early estimates of oil quantity and GOR for exploration targets in the area of interest.
Using Biomarkers in Oil to Assess Source Thermal Maturity
The relative abundances of certain biomarkers in petroleum change as a function of source rock maturity. As a result, a variety of biomarker parameters have been identified that are very useful for characterizing the source rock maturity simply from analysis of the migrated oil (e.g., Peters and Moldowan, 1993).
Biomarker maturity parameters (e.g., parameters such as those in Table 2) make use of processes that occur during source rock maturation:
- Cracking--large molecules break into smaller molecules
- Isomerization--changes in the 3-dimentional arrangements of atoms in molecules.
- Aromatization--formation of aromatic rings (loss of hydrogen from naphthenes)
Petroleum Biomarkers Indicative of Source Rock Maturity
Various considerations must be kept in mind when using petroleum biomarkers to assess source rock thermal maturity. For example:
- The exact relationship between a biomarker parameter and the source maturity is a function of heating rate, source lithofacies, and source organic facies (kerogen type). As a result, the exact maturity (i.e., vitrinite reflectance equivalent) associated with a given value for a biomarker parameter can change from basin to basin. Furthermore, the relationship between a biomarker maturity indicator and source rock maturity is generally non-linear.
- With increasing maturity, many biomarker maturity indicators reach terminal values; hence, a given biomarker parameter is applicable only over a specific maturity range.
- The concentrations of biomarkers in petroleum decrease with thermal maturity.
Despite these limitations, biomarker indicators of source maturity can be extremely useful. For example, biomarker maturity parameters can be used to determine what the API gravity of a biodegraded oil was prior to biodegradation. This is accomplished by collecting a suite of non-degraded oils from the same petroleum system as the degraded oils. Using the non-degraded oils, the geochemist develops a correlation or "transform" between a biomarker maturity parameter and API gravity. The same biomarker parameter is then measured on a degraded oil, and the original gravity is determined using the transform developed from the non-degraded oil suite. Moldowan, et al. (1992) provide an excellent example of this approach in which they determine the original gravity of degraded Adriatic oils. For this application, the most effective biomarker parameters are those based on compounds that are highly resistant to biodegradation, such as [Triaromatic/(Monaromatic +Triaromatic steroids)].
Source Rock descriptions and source rock maturity information derived from oil biomarkers are often key input data for basin modeling of a prospect or block.
Biomarkers are generally minor components of a crude oil or rock extract, and highly selective and sensitive methods of analysis employing gas chromatography – mass spectrometry (GC-MS) are necessary for their measurement. Typically a crude petroleum sample is separated into a saturate and aromatic fraction prior to biomarker analysis and then both fractions are analyzed independently. This provides greater resolution with less interference and allows for the determination of biomarker concentrations down to the low ppm and ppb levels. It is typical for a single analysis to monitor several hundred biomarkers. If additional selectivity is needed to resolve more complex biomarker distributions a tandem mass spectrometry technique (GC-MSMS) is used to analyze and distinguish biomarkers further.
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